This invention is in the field of electrooptical devices and liquid crystal materials used in such devices. More particularly, the invention relates to bookshelf-type liquid crystal materials and devices employing such liquid crystal materials.
Liquid crystals have found use in a variety of electrooptical and display device applications, in particular those which require compact, energy-efficient, voltage-controlled light valves such as watch and calculator displays.
Thermotropic liquid crystal molecules typically possess structures which combine a rigid core coupled with two relatively xe2x80x9cfloppyxe2x80x9d tails. Such LC molecules are generally rod-like in shape with the rigid core generally along the long axis of the molecule. Ferroelectric liquid crystal (FLC) materials have been prepared by the introduction of one or more chiral nonracemic LC molecules having one or more stereocenters in at least one of the tails to introduce chirality. The first FLC compound to be characterized was DOBAMBC which contains an (S)-2-methylbutyloxy chiral tail. Pure DOBAMBC exhibits a smectic C* phase with a ferroelectric polarization of xe2x88x923 nC/cm2.
Electro-optic effects with sub-microsecond switching speeds can be achieved using the technology of N. A. Clark and S. T. Lagerwall(1980) Appl. Phys. Lett. 36:899 and U.S. Pat. No. 4,367,924. These investigators have reported display structures using FLC materials, the so-called Surface-Stabilized FLC (SSFLC) devices, having not only high speed, but which also exhibit bistable, threshold sensitive switching. Such properties make FLC-based devices excellent candidates for light modulation devices including matrix addressed light valves containing a large number of elements for passive displays of graphic and pictorial information, optical processing applications, as well as for high information content dichroic displays.
It is, however, well known in the art of FLC materials and devices that a typical FLC device does not exhibit true optical bistability, that is, the memory or the zero applied field orientation of the optic axis of the SSFLC device is typically different from that of its driven orientation. Descriptions of the construction and operation of a conventional bistable FLC device can be found, for example, in U.S. Pat. Nos. 5,748,164 and 5,808,800. The FLC materials used in these conventional devices exhibit smectic layer spacing shrinkage at the smectic A to smectic C transition and further into the smectic C phase. The most significant consequence of the decrease in smectic layer thickness is the formation of chevron smectic layer structures. In addition to inducing many defects, formation of such chevron structures, in effect, adds an extra interface at the chevron interface which is a nominally planar interface roughly parallel to the plane of the FLC film. This extra interface is internal to FLC materials, and together with the two surfaces bounding the FLC materials and the external electric field, determines the orientation of the optic axis of the FLC device. The added constraint imposed by the chevron interface is that the orientation of the optic axis of the FLC devices under an applied electric field depends on the strength of the applied field, and is, thus, different from the memory orientation of the device in the absence of the applied field. See, for example, Rieker, T. et al. (1987) Physical Rev. Letts. 59(23):2658 for a discussion of chevron layer structure in SSFLC cells.
FIG. 1A schematically illustrates a typical electrooptical response (output light intensity as a function of applied voltage) of a conventional bistable FLC device. This conventional bistable device does not exhibit a true bistable switching and does not exhibit analog behavior. FLC compositions exhibiting bookshelf geometry will, in contrast, be substantially chevron-free when aligned in SSFLC devices and exhibit true bistable electrooptical response as schematically illustrated in FIG. 1B.
Much attention has focused on the construction of FLC electrooptical devices with true optically bistability which are extremely desirable in practical applications to achieve stable memory performance, high contrast ratio, wide viewing angle and high speed response. However, only a few FLC materials have been identified which exhibit true bistability. A small class of naphthalene-based LCs were reported to be useful for preparation of FLC mixtures exhibiting optical bistability (Mochizuki et al. (1991) Ferroelectrics 122:37-51, U.S. Pat. No. 5,169,556, EP published application 405,868 (published Feb. 1, 1991) and U.S. Pat. No. 5,348,685). These FLC materials are said to have bookshelf geometry and to exhibit no smectic layer spacing shrinkage at the smectic A (SmA) to the chiral smectic C (SmC*) transition and into the SmC* phase range, unlike many conventional FLC materials. U.S. Pat. Nos. 5,568,299, 5,856,815 and 5,943,112 report applications of the naphthalene-based FLCs of U.S. Pat. Nos. 5,169,556 and 5,348,685. Additional naphthalene-core LCs are reported to provide improvement in response times and/or temperature dependency of response time in U.S. Pat. No. 5,861,108.
U.S. Pat. Nos. 5,262,082, 5,437,812 and 5,482,650 report achiral LC compounds having perfluoroether terminal groups exhibiting smectic phases or latent smectic phases that are said to provide xe2x80x9creduced temperature dependence of the smectic interlayer spacingxe2x80x9d and xe2x80x9cspontaneous generation of a bookshelf layer structure ideal for a ferroelectric liquid crystal device.xe2x80x9d Preferred chiral LCs of these patents have a phenylpyrimidine core. A number of LC molecules have been reported to be useful in combination with these achiral bookshelf LCs.
U.S. Pat. Nos. 5,474,705, 5,702,637 and 5,972,241, as well as published EP application EP 736,078 (published Jun. 24, 1998) report chiral LC compounds also having a perfluoroether terminal portion or a chiral fluorinated terminal portion with preferred LC compounds having phenylpyrimidine cores. These patents report that the chiral LC molecules disclosed can be admixed with the achiral fluoroether-containing compounds of U.S. Pat. Nos. 5,262,082, 5,437,812 and 5,482,650 to exhibit xe2x80x9creduced temperature dependence of the smectic interlayer spacingxe2x80x9d and xe2x80x9cspontaneous generation of a bookshelf layer structure ideal for a ferroelectric liquid crystal device.
U.S. Pat. Nos. 5,658,491, 5,855,812 and 5,928,562 report a process for controlling cone tilt angle in tilted smectic FLC compositions. The compounds disclosed contain fluoroether or fluoroalkyl groups in the LC tail. The patents further report that the compounds useful in the invention can be admixed with the achiral fluoroether-containing compounds of U.S. Pat. Nos. 5,262,082, 5,437,812 and 5,482,650 to exhibit xe2x80x9creduced temperature dependence of the smectic interlayer spacingxe2x80x9d and xe2x80x9cspontaneous generation of a bookshelf layer structure.xe2x80x9d
U.S. Pat. Nos. 4,886,619, 5,082,587, 5,399,291, 5,399,701 report chiral and achiral LC molecules having tilted smectic mesophases or latent tilted smectic mesophases and having fluorocarbon terminal portions.
U.S. Pat. Nos. 5,750,214 and 5,858,273 report liquid crystal devices with certain alignment control, which is said to be useful in improving a switching characteristic of a chiral smectic liquid crystal composition having bookshelf structure. The patents refer to the use of FLC compositions in the method in which at least one component of the FLC composition has a fluorocarbon terminal portion. The patents refer specifically to the use of compounds of bookshelf LCs of U.S. Pat. No. 5,262,082.
U.S. Pat. Nos. 6,019,911 and 6,007,737 report various liquid crystal compositions having structures related to the naphthalene and phenyl pyrimidines that are noted above to exhibit spontaneous generation of bookshelf structure. However, none of the LC compounds disclosed in these patents is identified as exhibiting bookshelf structure or as useful in the preparation of chevron-free FLCs.
Various naphthalene-containing liquid crystals of general formula: 
where M, N and P are various ring structures; A, B and D are linkers; R and Rf are tails and X, Y and Z are substitutents such as hydrogen, halogen, hydroxyl, methyl, methoxy, cyano or nitro; are reported in U.S. Pat. No. 5,972,241 (Johnson, Oct. 26, 1999) U.S. Pat. No. 5,702,637 (Dec. 30, 1997) U.S. Pat. No. 5,082,587 (Janulis, Jan. 21, 1992); U.S. Pat. No. 5,417,883 (Epstein, May 23, 1995); U.S. Pat. No. 5,399,291 (Janulis, Mar. 21, 1995); U.S. Pat. No. 5,858,273 (Asaoka, Jan. 12, 1999); U.S. Pat. No. 5,262,082 (Janulis, Nov. 16, 1993); U.S. Pat. No. 5,482,650 (Janulis, Jan. 9, 1996); U.S. Pat. No. 5,437,812 (Janulis, Aug. 1, 1995); U.S. Pat. No. 4,886,619 (Janulis, Dec. 12, 1989); U.S. Pat. No. 5,399,701 (Mar. 21, 1995); U.S. Pat. No. 5,254,747 (Janulis, Oct. 19, 1993); U.S. Pat. No. 5,474,705 (Janulis, Dec. 12, 1995); U.S. Pat. No. 5,254,747 (Janulis, Oct. 19, 1993); U.S. Pat. No. 5,082,587 (Janulis, Jan. 21, 1992); WO 00/31210 (Minnesota Mining and Manufacturing Company, published Jun. 2, 2000); European Patent 736078 (June, 1998); European Patent 255236 (Minnesota Mining and Manufacturing Company, May, 1994); WO 99/33814 (Minnesota Mining and Manufacturing Company, publication date Jul. 8, 1999); U.S. Pat. No. 5,928,562 (Kistner, Jul. 27, 1999); U.S. Pat. No. 5,855,812 (Radcliffe, Jan. 5, 1999); U.S. Pat. No. 5,658,491 (Kistner, Aug. 19, 1997); These patents and applications do not disclose bistable hosts.
U.S. Pat. Nos. 6,057,007, 6,084,649, report tristable liquid crystal devices comprising a tilted smectic or induced tilted smectic LC composition. Many of the LC molecules specifically exemplified have phenylpyrimidine cores and a chiral or achiral terminal fluorocarbon group.
There are two types of bookshelf materials known to date: naphthalene and partially-fluorinated poly-ether derivatives. Naphthalene derivatives have been reported to be easier to align than partially-fluorinated poly-ether derivatives but they typically have high viscosity and low switching angle (below 18xc2x0) which result in slow switching speed, low light throughput, and low A-C* transition. These disadvantages limit the color depth, contrast and brightness of the display and result in narrow operating and storage temperatures.
Bookshelf liquid crystal compositions that have lower viscosity, higher tilt angle, and higher A-C* transition than previously known compositions to minimize the drawbacks of chevron-type FLCs and allow DC balanced driving schemes that maximize light throughput are desired.
The present invention provides electrooptical devices that contain bistable bookshelf liquid crystals. Provided are liquid crystal compositions comprising a bistable host material and one or more compounds of formula: 
where R1 and R4 are selected from the group consisting of: straight chain alkyl chains and internally or terminally branched alkyl chains with from 3 to 12 carbons which are optionally partially fluorinated and may have one carbon replaced with xe2x80x94Oxe2x80x94;
where A is a cyclohexyl ring, a cyclohexenyl ring or a phenyl ring;
D is either O or 
xe2x80x83and the D""s may be different;
a is 0 or 1;
b is 0, 1 or 2;
c is 0 or 1;
d is 0 or 1;
R5 is a fluorine atom;
R2 is a linker selected from the group consisting of: ethyl; butyl; 
xe2x80x83and xe2x80x94Oxe2x80x94C(xe2x95x90O)xe2x80x94; and
E is a naphthalene ring or a 1,2,3,4-tetrahydronaphthalene ring 
As used herein, xe2x80x9cpartially fluorinatedxe2x80x9d means that one or more hydrogens are replaced with a fluorine. xe2x80x9cPartially fluorinatedxe2x80x9d also means that a hydrogen may be replaced with xe2x80x94CF3. As used herein, xe2x80x9cbranchedxe2x80x9d includes internally or terminally branched.
The xe2x80x9ccompounds of the inventionxe2x80x9d are those compounds with the general formula above, as well other compounds described herein. The xe2x80x9ccompositions of the inventionxe2x80x9d are those compositions comprising one or more compounds of the invention in a bistable host.
Also provided are the compounds disclosed herein that are not previously known in the art.
One group of compounds of the invention are those of the general formula above, wherein one of R1 or R4 is (CH2)n(CF2)mF where n is an integer from 1 to 6 and m is an integer from 1 to 6. Another group of compositions of the invention are those wherein said bistable host material contains at least one naphthalene-containing compound. Another group of compositions of the invention are those with a switching angle greater than 15xc2x0. Another group of compositions of the invention are those with an A-C transition temperature greater than 50xc2x0 C. Another group of compositions of the invention are those aligning in a device with a contrast ratio greater than 20:1. One group of compounds of the invention comprise those wherein the naphthalene-containing liquid crystal includes a partially fluorinated alkoxy tail. One group of compounds of the invention are those where A is a cyclohexyl ring. Another group of compounds of the invention are those where A is a cyclohexenyl ring. Another group of compounds of the invention are those where A is a phenyl ring.
One presently preferred group of compounds useful in the compositions of the invention are those containing a compound of formula (A): 
where R1 and R4 are selected from the group consisting of: straight chain alkyl chains and branched alkyl chains with from 3 to 12 carbons which are optionally partially fluorinated and may have one carbon replaced with xe2x80x94Oxe2x80x94;
D is either O or 
xe2x80x83or xe2x80x94Oxe2x80x94C(xe2x95x90O)xe2x80x94 and the D""s may be different;
a is 0 or 1;
b is 0, 1 or 2;
c is 0 or 1;
d is 0 or 1;
R5 is a fluorine atom;
R2is a linker selected from the group consisting of: ethyl; butyl; xe2x80x94Oxe2x80x94C(xe2x95x90O)xe2x80x94; and 
Presently preferred groups of compounds of the formula A are those wherein R1 is a partially fluorinated straight chain alkyl group and R4 is a nonfluorinated straight chain alkyl group and R2 is an ethyl group; those wherein R1 is an internally branched alkyl chain and R4 is a partially fluorinated alkyl chain and R2 is an ethyl group; those wherein R1 is a partially fluorinated straight chain alkyl chain and R4 is an alkyl chain where one of the carbons of the chain is replaced with O and another of the carbons of the chain is substituted with xe2x80x94CF3 or xe2x80x94CH3; and those wherein R1 is a partially fluorinated straight chain alkyl group and R4 is a nonfluorinated straight chain alkyl chain.
Another presently preferred composition of the invention comprises a bistable host material and a compound of formula (B): 
where R1 and R4 are selected from the group consisting of: straight chain alkyl chains and branched alkyl chains with from 3 to 12 carbons which are optionally partially fluorinated and may have one carbon replaced with xe2x80x94Oxe2x80x94;
D is either O or 
xe2x80x83or xe2x80x94Oxe2x80x94C(xe2x95x90O)xe2x80x94 and the D""s may be different;
a is 0 or 1;
b is 0, 1 or 2;
c is 0 or 1;
R5 is a fluorine atom;
R2 is a linker selected from the group consisting of: ethyl; butyl; 
xe2x80x83and xe2x80x94Oxe2x80x94C(xe2x95x90O)xe2x80x94; and
Presently preferred compounds of formula B include those wherein R1 is a partially fluorinated straight chain alkyl group and R4 is an internally branched nonfluorinated alkyl chain.
Also provided are compounds of formula (C): 
where R1 and R4 are selected from the group consisting of: straight chain and internally or terminally branched alkyl chains with from 3 to 12 carbons which are optionally partially fluorinated and may have one carbon replaced with xe2x80x94Oxe2x80x94;
D is either O or 
xe2x80x83or xe2x80x94Oxe2x80x94C(xe2x95x90O)xe2x80x94 and the D""s may be different;
a is 0 or 1;
b is 0, 1 or 2;
c is 0 or 1;
R5 is a fluorine atom;
R2 is a linker selected from the group consisting of: ethyl; butyl; 
xe2x80x83and xe2x80x94Oxe2x80x94C(xe2x95x90O)xe2x80x94.
A presently preferred composition of the invention comprises a bistable host material and a compound of formula: 
A presently preferred composition of the invention comprises a bistable host material and a compound of formula: 
A presently preferred composition of the invention comprises a bistable host material and a compound of formula: 
A presently preferred composition of the invention comprises a bistable host material and a compound of formula: 
A presently preferred composition of the invention comprises a bistable host material and a compound of formula: 
A presently preferred composition of the invention comprises a bistable host material and a compound of formula: 
A presently preferred composition of the invention comprises a bistable host material and a compound of formula: 
A presently preferred composition of the invention comprises a bistable host material and a compound of formula: 
A presently preferred composition of the invention comprises a bistable host material and a compound of formula: 
A presently preferred composition of the invention comprises a bistable host material and a compound of formula: 
A presently preferred composition of the invention comprises a bistable host material and a compound of formula: 
A presently preferred composition of the invention comprises a bistable host and a compound of formula MDW 1495 shown below.
Compositions of the invention contain one or more compounds disclosed herein in a bistable host, resulting in a composition that exhibits the desired properties. The compounds added to the bistable host may be present in the composition at a concentration of between about 2 and 100% by weight and all intermediate ranges therein, including between about 2% to about 35% by weight; between about 5% to about 20% by weight; between about 5% to about 50% by weight; and about 10% by weight. Preferably, the compound or compounds added to the bistable host are present in the composition at between about 5% and 15% by weight.
Provided are bistable FLC devices which comprise about 2% to about 100% by weight of one or more compounds of the invention in a bistable host material, and all intermediate ranges therein, including those ranges listed above. Napthalene-containing liquid crystals and non-naphthalene-containing liquid crystals may be present in the same device, providing that the device has the desired characteristics. These device may have other components as known in the art, as long as they do not interfere with the desired characteristics of the device. Preferably, the liquid crystal added to the bistable host is selected from the group consisting of: 
The bistable FLC devices of the invention have a switching angle greater than 15xc2x0; an A-C transition higher than 50xc2x0 C.; and align with a contrast ratio greater than 20:1.